Focus determining device for photographic enlargers



Jan., 3, 1950 w. D. THOMPSON FOCUS DETERMINING DEVICE FOR PHOTOGRAPHIC ENLARGERS Filed Oct. 24, 1946 N. w M m 0. WM/ Y f ATTOR/Vfv Patented Jan. 3, 195@ FOCUS DETERMINING DEVICE FOR PHOTOGRAPHIC ENLARGERS William D. Thompson, San Diego, Calif.

Application October 24, 194.6, Serial No. 705,442

3 Claims. l

This invention relates to a focus determining device for photographic enlargers, and has for its principal object the provision of a small, compact, easily used instrument by means of which it may be ascertained when a photographic enlarger is in perfect focus with the enlarging screen.

The invention is more particularly designed for use on the enlarging apparatus illustrated and described in applicants co-pending application, Serial No. 705,441, now abandoned. It is, however, not limited to this particular use but Will be found valuable with any conventional enlarging apparatus. y

Other objects and advantages reside in the detail construction of the invention, which is designed for simplicity, economy, and eiciency. These will become more apparent from the following description.

In the following detailed description of the invention, reference is had to the accompanying drawing which forms a part hereof. Like numerals refer to like parts in all views of the drawing and throughout the description.

In the drawing:

Fig. l is a vertical section through the projection head of a conventional photographic enlarging projector, illustrating the invention in place therein;

Fig. 2 is a side view of the improved focus determining device;

Fig. 3 is a front view thereof;

Fig. 4 is a horizontal section, looking upwardly on the line 4-4, Fig. 2;

Fig. 5 is a detail view of a removable door, the removal of which allows the improved focus determining device to be inserted in the enlarger;

' and Fig. 6 is a face view of a focusing target which may be used with the improved enlarger.

The improved focus determining device is to be used with any enlarger having a light source, a film holder, and a projecting lens system and is designed to be placed between the light source and the lm holder.

In Fig. l the typical parts of a conventional enlarging apparatus are illustrated and designated by numeral as follows: condenser lens barrel ID; lens head chamber II; objective lens collar I2; objective lens barrel I3; objective lenses I4; and condensing lenses I5. The apparatus illustrated is provided with a film holder I6 of any desired type in which the lm to be enlarged is indicated at I1.

In the enlarging device illustrated, the chamber II is provided with a doorway in its side normally closed by a removable door I8, shown in detail `in Fig. 5. The door is normally held in place by means of snap springs I9 and can be removed simply by pulling with sufficient force to release the springs I9 from the doorway. When in use the improved focus determining attachment replaces the door I8.

In use, the enlarger is brought to an approximate focus by means of the eye of the operator. The door I3 is then removed and the focusing device is placed in the dooropening to determine the nal focus.

The focus determining device carries a door plate 2l) of a size and shape to occupy the position of the door I8 so as to seal the chamber II. The door plate 20 is provided with snap springs 2| which engage the edges of the door opening to hold the plate 20 in place thereon.

A hollow housing 22 projects inwardly from the door plate to provide a guide for a lens barrel 23. The lens barrel is slidably mounted in a guide partition 24 in the housing 22 and is threaded through the door plate 20 so as to prt I ject outwardly therefrom. An annular locking nut 25 is threaded on the barrel 23 to lock it in any desired position in the door plate 20.

The lens barrel 23 is provided with objective lenses 26 adjacent its inner extremity and carries an eye piece 21 threaded into its outer extremity. The eye piece carries a magnifying lens 28 which cooperates with the lenses 26 to project a magnified image into the eye of the operator. The image is received from a reflecting device, which may be a mirror or a prism. As illustrated, an inclined mirror 29 is used, preferably vsilvered on its front face.

The mirror 29 is supported at an angle of 45 from the vertical by means of two bracket arms 30 extending from the end of the housing 22. The bracket arms 30 support the mirror 29 at the optical axis of the objective and condensing lenses I4 and I5, and at a position above the film I1, that is, between the film and the light source of the enlarger.

If the enlarging objective lenses I4 are out of focus with the enlarging screen, the mirror 29 will reflect two images into the eye of the user. If these lenses are in perfect focus with the screen, only a single image will be reflected. One of these images consists of a picture of the film itself, for since the back of the lm is brilliantly illuminated, a perfect image thereof will be reflected and projected in magnified form to the operators eye. The other image is due to the fact that the objective lenses i4 will act in the nature of camera lenses and throw an image of the illuminated picture, which is being projected upon the enlarging easel, back upon the film I1, which acts as a translucent projection screen.

In other words, if a screen were positioned at the place of the lm and an illuminated picture were placedupon the. enlarging easel', a. reduced image of' thispicture would be thrown upon:V the screen. The same thing occurs upon the lm when the film I7 occupies the position of the;

screen. The picture in the latter case consists of the illuminated image projected.. by. the: errlarger. rThis projected image will only coincide objective lenses I4 are in perfect focus. This can be easily determined by the operator, since the lenses 26 and 28 greatly magnify thefdoubles image on the lm.

The door plate 20 and the snap. springs 2| are simply illustrativeof one method. off detachably insertingV the focusing device inthe light.. cone of anenlargingdevice. Other means could be em.- ployed. for detachably inserting the device, depend-ing, of course, upon the particular. structure of the. enlarging- ,deviceinquestion. The principal feature of the invention resides in providing means for projecting an. image lof the film. to one side of the optical axis of the enlarging. device so; that a. comparison may be made between the actual emulsion picture and the returned pre.- jected picture thereof.

While the improved focus determining device has: been illustrated. and described as removable, itlcould be. built inpermanently, if desired. Tests made with a 1/2. x1/2 mirror and a foot-candle meter show but a slight deviation of the indicating needle and no difference isA discernible with vthe eye in the light striking the easel `when the .l

focusing. device is removed and replaced. However, since the device is no longer needed, after the-focusis set,.it can be removed, if desired..

For thin negatives, or negatives. withoutcontrasty detail, focusingis` facilitated by means lof a focusing target. 3i. (see Fig. 6.-), `which maybe placed onther enlarging board. or easel. The clear areas of the filmznegative then act as a, ground glass upon which the image of. the target, is :focused by means of this improved. `focusing de,- vice. The target shouldbe relatively large, say 8 x 10".

The threads 0n the lens barrel 23 are for the purpose of. -focusing the magnifying. lenssystem onthe negative with the eyes of the individual .l

user to obtain the most, perfect focus possible. After being focused and. set by means of: the nut 25 itwill remainfixed as long as the same `person uses the. device. This., particular focusing has nothing t0 do with the focusing of.. the enlarger. Itlisonly. for obtaining ther best results with each user.

While. aspecific form of the improvement. has been `described and. illustrated herein, it is. de.- siredl to be understood that the same may be varied, within the scope ofthe appended claims,

without departingfrom the spirit of the invention. Having thus described thel invention, what? is claimed and desired secured by Letters Patent is: 1. A focus determining device for `photographic enlargers comprising: condensing. lenses; objec` tive lenses, all `of said lenses beingy positioned. in axial alignment so that said condensing lenses will refract a conical beamy lof light through said objective lenses; a photographic film having a developed image interposed in said beam of light between said condensing and said objective lenses -with its front surface facing said objective lenses so that Ian illuminated picture `of said image will be projected upon an enlarging easel; a reflecting device positioned in said beam of light between said condensing lenses and said nlm and having its' reecting surface directed toward said film at an angle. as to reflect light from the rear surface of said lm outwardly at a substantially 90 axis from the optical axis of said lenses; and means for. viewingthe rear surface of said film as reflectedfrom. said reflecting surface.

2. A focus determining device for photographic enlargerscomprising: condensing lenses; objective lenses, all of said lenses being positioned in axial alignment' so that said condensing lenses willrefract a conical beam of light through said objective lenses; a photo-graphic film having a developed. image. interposed in said. beam of light betweensaid condensing andsaid objective lenses with its, front surface facing said objective. lenses so that. an illuminated picture of said image will be projected upon an enlarging easel; a mirror positicnedin saidcone. of light between said condensing lenses and said lm, said mirror being substantially. smaller than the light beam so as to intercept only a relatively smallffraction `of the light and having its reflecting surface directed toward.. said film at an angle vbeing to reil'ect an imagev of a.. portion of the rear side of said film outwardlyA fromsaid optical axis;.and magnifying lenses positioned to receive the light reiiected fromY said mirror for. viewingsaid rear side of said film.

3. The combination in a photographic enlarger of a lens chamber; condensing lenses 4positioned at one extremity of' said barrel; objective lenses Ipositioned at, the other extremity of said chamber in axial alignment ywith said. condensing lenses; almholder positioned in said chamber between said condensing and objective lenses; a photographic lm insaidholder; a lens barrel extending radially into said chamber through the wall thereof between said. condensing lenses and said holder; viewing. lenses in. said lens barrel; a mirror; and` bracket meanssupporting saidV mirror substantially at the intersection of the optical axis of said condensing and objective lenses and the optical axis of said viewing lenses, said mirror being positioned .at an angle Vof 45 to both said axes andfacing said holder so as to reflect light from said fllm through said viewing lenses.

WILLIAM D. THOMPSON.

REFERENCES CITED The. followingl referencesare of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,276,733,- Da-Vs Aug. 27, 1918 2,089,703 May` Aug. 10,` 1937 2,282,045 Fleischer May-5,1942

FOREIGN PATENTS' Number Country Date 15,199 Great Britain July 2, 1907 OTHER REFERENCES.

Book',` Optical Measuring Instruments, Martin, 1924, page 187; (Copy i'n Division 7.) 

